In Titan's Curse, Percy & Co. track down Nereus, the old man of the sea, so they can shake him up for some answers. When they pass by him in Son of Neptune, Nereus points one bony finger at Percy and mouths, "Don't even think about it."
Frank bashing Alcyoneus repeatedly on the nose with his shield to keep him unconscious while they drag him out of Alaska.
The video-scroll message that is almost at the ending of "Son Of Neptune" can count as funny.
Leo: What's my title? Am I like admiral, or captain, or...
Piper: Repair Boy.
Leo: Very funny, Piper.
This exchange in The Son of Neptune when Hazel and Frank explain the war games to Percy:
Hazel: "No one dies. Well, not usually And if they do-"
Frank:(imitating Vitellius) "They're wimps! Back in my day, we died all the time, and we liked it!"
And this one, between Frank and Percy:
Frank: Can your sword do grenade form?
Percy: No. Shut up.
"Die, mosquito! Die, mosquito!"
"Spears are good for subtraction."
Percy is mad because Poseidon gave the Zhangs the power to change shape while he can't, rendering Frank nearly speechless given the terrifyingly awesome powers Percy has revealed; giant fists of Tiber water, exploding water cannon, mini-hurricanes...
Percy's reaction to accidentally finding Polybotes leading the invasion army south toward New Rome.
Percy: "Schist!"
Anything to do with Schist.
"Please don't let there be an app for reaping souls."
"Percy hated tests. Since he lost his memory, his entire life felt like one big fill-in-the-blank. He was __ from __, he felt like __, and if the monsters caught him, he'd be __."
A conversation between June and Percy:
June: All roads lead there, child. You should know that.
Percy: Detention?
Percy asks a whale to carry them to Alaska. The whale asks if they're seals, and Percy replies, 'No, but I've got a man satchel full of macrobiotic beef jerky.'
Promise not to feed me this, and I will take you north.
Pretty much anything to do with Percy's rainbow man-satchel, actually.
"O Fleecy, do me a solid. Show Reyna at Camp Jupiter."
Iris calls the people who work in her store ROFLcopters.
Arion the foul-mouthed horse.
As in the first series, half the things Tyson is involved in. One gem in particular, involving Ella:
Ella: The groin. The Earthborn's groin is sensitive.